Add CFData::from_arc

This allows creating a zero-copy CFData wrapping existing data.

core-foundation/src/data.rs

@@ -14,6 +14,8 @@ use core_foundation_sys::base::CFIndex;
 use core_foundation_sys::base::{kCFAllocatorDefault};
 use std::ops::Deref;
 use std::slice;
+use std::sync::Arc;
+
 
 use base::{CFIndexConvertible, TCFType};
 
@@ -26,6 +28,7 @@ impl_TCFType!(CFData, CFDataRef, CFDataGetTypeID);
 impl_CFTypeDescription!(CFData);
 
 impl CFData {
+    /// Creates a CFData around a copy `buffer`
     pub fn from_buffer(buffer: &[u8]) -> CFData {
         unsafe {
             let data_ref = CFDataCreate(kCFAllocatorDefault,
@@ -35,6 +38,41 @@ impl CFData {
         }
     }
 
+    /// Creates a CFData referencing `buffer` without creating a copy
+    pub fn from_arc<T: AsRef<[u8]> + Sync + Send>(buffer: Arc<T>) -> Self {
+        use std::os::raw::c_void;
+        use crate::base::{CFAllocator, CFAllocatorContext};
+
+        unsafe {
+            let ptr = (*buffer).as_ref().as_ptr() as *const _;
+            let len = (*buffer).as_ref().len().to_CFIndex();
+            let info = Arc::into_raw(buffer) as *mut c_void;
+
+            extern "C" fn deallocate<T>(_: *mut c_void, info: *mut c_void) {
+                unsafe {
+                    drop(Arc::from_raw(info as *mut T));
+                }
+            }
+
+            // Use a separate allocator for each allocation because
+            // we need `info` to do the deallocation vs. `ptr`
+            let allocator = CFAllocator::new(CFAllocatorContext {
+                info,
+                version: 0,
+                retain: None,
+                reallocate: None,
+                release: None,
+                copyDescription: None,
+                allocate: None,
+                deallocate: Some(deallocate::<T>),
+                preferredSize: None,
+            });
+            let data_ref =
+                CFDataCreateWithBytesNoCopy(kCFAllocatorDefault, ptr, len, allocator.as_CFTypeRef());
+            TCFType::wrap_under_create_rule(data_ref)
+        }
+    }
+
     /// Returns a pointer to the underlying bytes in this data. Note that this byte buffer is
     /// read-only.
     #[inline]
@@ -61,3 +99,46 @@ impl Deref for CFData {
         self.bytes()
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::CFData;
+    use std::sync::Arc;
+
+    #[test]
+    fn test_data_provider() {
+        let l = vec![5];
+        CFData::from_arc(Arc::new(l));
+
+        let l = vec![5];
+        CFData::from_arc(Arc::new(l.into_boxed_slice()));
+
+        // Make sure the buffer is actually dropped
+        use std::sync::atomic::{AtomicBool, Ordering::SeqCst};
+        struct VecWrapper {
+            inner: Vec<u8>,
+            dropped: Arc<AtomicBool>,
+        }
+
+        impl Drop for VecWrapper {
+            fn drop(&mut self) {
+                self.dropped.store(true, SeqCst)
+            }
+        }
+
+        impl std::convert::AsRef<[u8]> for VecWrapper {
+            fn as_ref(&self) -> &[u8] {
+                &self.inner
+            }
+        }
+
+        let dropped = Arc::new(AtomicBool::default());
+        let l = Arc::new(VecWrapper {inner: vec![5], dropped: dropped.clone() });
+        let m = l.clone();
+        let dp = CFData::from_arc(l);
+        drop(m);
+        assert!(!dropped.load(SeqCst));
+        drop(dp);
+        assert!(dropped.load(SeqCst))
+    }
+}